In the prior art, lower carbon, high strength (or High Performance Steel, HPS) weathering grade steels are being increasingly employed for bridge, pole and other high strength applications. These steel materials offer three advantages over concrete and other types of steel materials. First, the use of higher strength materials can reduce the overall weight of the structure being built and can also reduce the material cost. Consequently, designs using these weathering grade steels can be more competitive with concrete and those designs employing lower strength steels. Second, the weathering grade or atmosphere corrosion-resistant grade steel can significantly reduce the maintenance cost of structures such as bridges or poles by eliminating the need for painting. These weathering grad e steels are particularly desirable in applications which are difficult to regularly maintain, for example, bridges or poles located in remote areas. Third, lower carbon (i.e., 0.1% carbon maximum) and lower carbon equivalent levels improve the weldability and toughness of the steel.
The use of th ese types of steels is guided by ASTM specifications. One ASTM specification for a weathering grade steel which is commonly used for bridge applications includes A709-Grades 70W and HPS 70W. The bridge-building, 70W grades require a 70 KSI minimum in yield strength. This specification also requires that these grades be produced by rolling, austenitizing, quenching, and tempering. The conventional 70W grade is a higher carbon grade (0.12% by weight), whereas the newer HPS 70W grade utilizes a lower carbon level (0.10% by weight). The HPS 70W grade is generally produced in plates up to 3.0" in thickness. Table 1 lists the ASTM specifications with Table 2 detailing the mechanical property requirements for the various specifications. Table 3 details the compositional requirements for these specifications. The disclosure of ASTM specification number A709 for all grades is hereby incorporated by reference. As noted above, the higher strength specifications require a hot rolled, austenitized, quenched, and tempered processing. Moreover, the tensile strength is specified as a range, i.e., 90-110 KSI, rather than a minimum which is used in other specifications, see for example, A871-Grade 65 that specifies a tensile strength greater than or equal to 80 KSI.
ASTM weathering grade plate specifications are not without their disadvantages. First, processing whereby the hot rolled product must be reheated, quenched and tempered is energy intensive. Second, these quenched and tempered grades are limited by plate length due to furnace length restrictions. In other words, only certain length plates can be heat treated following the quenching operation since the furnaces will accept only a set length, in some instances, only up to 600". Bridge builders particularly are demanding ever-increasing lengths (to reduce the number of splicing welds required and save fabrication cost) of plate for construction, such demands are not being met by current plate manufacturing technology for high strength steels.
Many bridge manufacturers are also requiring thicker plates for more-demanding applications. Present day prior art grades do not always offer a cost-effective solution when thick plates, e.g., greater than 2" or even as thick as 3" are desired.
Third, the high strength ASTM specifications requiring a minimum of 70 KSI yield strength also poses a difficulty in manufacturing by specifying a lower and an upper limit for tensile strength, i.e., 90-110 KSI for A709-Grade 70W. More particularly, one cannot merely target a minimum 70 KSI yield strength to meet the A709 specification since too high of a yield strength may also result in a tensile strength above the 110 KSI maximum.
In view of the disadvantages associated with current weathering grade steel specifications, a need has developed to produce plates in ever-increasing lengths and in a more cost-effective manner (lower production costs and quicker delivery). In addition, a need has developed to provide an as-rolled and cooled plate product having a greater thickness than presently available.
In response to the above-listed needs, the present invention provides a method of making a weathering grade steel plate and a product therefrom. More particularly, the inventive method uses a controlled alloy chemistry, a controlled rolling, and a controlled cooling to produce an as-rolled and cooled weathering grade steel plate which meets ASTM specification requiring a minimum of 70 KSI yield strength, a 90-110 KSI tensile strength, and good toughness when measured by Charpy V-notch impact energy testing. The inventive method combines controlled rolling and accelerated cooling with the controlled alloy chemistry to meet the ASTM specifications for 70 KSI minimum yield strengths, tensile strength of 90-110 KSI, toughness values of greater than 35 ft-lbs. at -10.degree. F., and plate up to 4.0" thick. The processing is more energy efficient since no re-austenitizing and tempering are required. Further, plates as thick as 3.0 to 4.0" can be manufactured while still meeting specification requirements.
The use of accelerated cooling and hot rolling is disclosed in U.S. Pat. No. 5,514,227 to Bodnar et al. (herein incorporated in its entirety by reference). This patent describes a method of making a steel to meet ASTM A572, Grade 50, a 50 KSI minimum yield strength specification. The alloy chemistry in this patent specifies low levels of vanadium and 1.0 to 1.25% manganese. Bodnar et al. is not directed to weathering grade steels nor methods of making plate products requiring either a yield strength in the range of 70 KSI, a tensile strength of 90-110 KSI, or a toughness value as stated above.